How do I get a driver variable to taper down to 0 once the variable is doubled?
In order to get the value moving back down towards zero once the bound has been reached you simply need to specify an expression that gives the desired result. The key is to think about the boundary cases - in this case, what happens at 0.26666
So, up to 0.26666 you want the output to be var
Above 0.26666 you want it to move back towards 0 - so you need to subtract var ie, -var, BUT, at 0.26666 you want it to equal 0.26666... ie, you want it to be (0.26666*2) - var
This would result in the expression var if var &lt; 0.26666 else 0.53332 - var.
You can actually link multiple of these conditionals together so you may want it to stay at zero once it reaches zero. ie,

below 0.26666 : var

0.26666 -&gt; 0.53332 : 0.53332 - var

over 0.53332 : 0


This could be produced with :
var if var &lt; 0.26666 else 0.53332 - var if var &lt; 0.53332 else 0